The present invention relates generally to a moisture control device for controlling the moisture existing inside a video printer so as to prevent deterioration of the printing quality.
Generally, when recording a video signal on a recording paper, sublimated thermal transcription is employed, in which the hue gradations may be freely and precisely expressed and therefore a full color recording may be achieved.
A known recording mechanism of the sublimated thermal transcription employed in a video printer, includes a thermal print head (T.P.H), a color ribbon, a recording paper, and a platen drum, wherein the recording paper is rotated three times during which the three dyes of yellow (Y), magenta (M) and cyan (C) coated on the color ribbon are thermally sublimated depending upon the magnitude of a video signal applied to the T.P.H, thus deposited on the recording paper. In this conventional process, the rise of the surface temperature of the T.P.H. considerably affects the amount of the heat radiated from the heating bodies of the T.P.H., and therefore the surface temperature should be detected to correct the temperature, as will be described with reference to FIG. 1 and FIGS. 2A-2C.
Referring to FIG. 1, the input video signal is converted into a digital signal by analog/digital converter 10 to be delivered as the output to a hue gradation controller 20 that controls the amount of the heat radiated from the heating bodies of the T.P.H. depending upon the magnitude of the input video signal. In this case, the surface temperature of the base of the T.P.H. 30 is detected serving as the reference signal for controlling the amount of the heat radiated. Further, since the surface temperature of the base of the T.P.H. 30 is high, a fan motor is driven to lower the surface temperature.
Referring to FIG. 1 and FIGS. 2A-2C, there is illustrated the operations and disadvantages of the conventional video color printer. In particular, FIG. 2B illustrates the changes of the recording concentration with relation to the surface temperatures of the base of the T.P.H. 30, in which a line 31 represents a characteristic of the conventional video color printer and a line 33 represents an ideal characteristic. In other words, as the operation time increases the heat radiated from the T.P.H. 30 will accumulate, which causes the printing error.
In FIG. 2C, there is described the changes of the surface temperature of the base of the T.P.H. 30 with respect to the recording time in case of an A6 size sheet, for example. Thus, it will be noted that an increase in of the surface temperature of the base of the T.P.H. directly affects the recording concentration. Hence, the amount of the heat radiated from the T.P.H. 30 is controlled in accordance with the result of the temperature detection by controlling the pulse width modulation (PWM) signal detected at the T.P.H. 30 to correct the temperature.
In such a conventional video printer, only the surface temperature of the base of the T.P.H. is corrected, and the influences of moisture are never taken into account. Hence, if the video printer works in an environment having excess moisture, fine particles of water migrate through the surfaces of the color ribbon, the recording paper, the platen drum, as well as inside the printer which directly affects the printing quality, so that the recording paper is not only slid when supplied to the printer, but also the printing quality is considerably affected.